Optical time division multiplexing

ABSTRACT

A method of multiplexing optical signals in a node of an optical network including as inputs a plurality of electrical signals, a plurality of laser transmitters, and as outputs a plurality of optical fibers, (a) generating clock pulses as a first clock frequency; (b) dividing the clock pulses respectively into a number of parallel trigger outputs; (c) sampling the electrical signals respectively by triggering on the parallel trigger outputs; (d) converting the sampled electrical signals to sampled optical signals by modulating respectively the laser transmitters with the sampled electrical signals and outputting respectively the sampled optical signals on the optical fibers; (e) combining the sampled optical signals onto a single optical fiber.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to optical communications and, moreparticularly, to a novel method of multiplexing optical signals onto asingle optical fiber. Specifically, the method includes samplingmultiple signals and coupling the sampled signals onto a single opticalfiber. The optical multiplexing method, of the present invention, usefulfor both digital and RF analog signals, is applicable and for upgradingexisting fiber optic communications networks and in new installations.

Multiplexing in optical fiber communications systems may be achieved byseveral existing technologies. The choice of multiplexing technology hasa tremendous impact on the cost, performance and the upgradeability ofthe network. One multiplexing technology is wavelength divisionmultiplexing (WDM). In WDM, data signals are each modulated on aseparate wavelength of light and then coupled onto a single opticalfiber. At the remote end of the optical fiber, the different wavelengthsare optically separated and individually detected. WDM systemsconsequently require a large number of laser sources and detectors foreach different wavelength.

An alternative to WDM that achieves high data rates on a singlewavelength channel is known as optical time division multiplexing(OTDM). A typical OTDM system as shown in the diagram of FIG. 1 includesa single high speed pulsed laser 101. An optical splitter 103 splits thepulsed laser output. An external modulator 109 is used to modulate eachsignal onto an optical carrier. Optical delay lines 105, such as fibersof varying lengths, are used to inter-leave the pulsed signals of thedifferent channels. The optical signals are then optically multiplexedby a combiner 111 onto a single optical fiber.

During the lifetime of fiber optic communications networks, there areoften situations in which additional fiber is required but not availableand upgradeability options are limited. Therefore, a method to easilyupgrade a fiber optic communications network installed in the fieldwould be advantageous. Current multiplexing technologies are not readilyupgraded once they are installed. For instance, a fiber opticcommunications network that does not include WDM lasers installed cannotbe upgraded using WDM without replacing all the laser transmitters andinstalling wavelength demultiplexers and/or filters at the receiver end.Upgrading a prior art OTDM network would require the installation ofoptical delay lines of precise delay values, a difficult procedureperformed in the field.

There is thus a need for, and it would be highly advantageous to have amethod of optical time division multiplexing by signal sampling whichcan be used to free optical fibers in an existing network that isstraightforward to install and requires replacing a minimum number ofexisting network components.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofmultiplexing optical signals in a node of an optical network includingas inputs a plurality of electrical signals, a plurality of lasertransmitters, and as outputs a plurality of optical fibers, includingthe steps of: (a) generating clock pulses at a first clock frequency;(b) dividing the clock pulses respectively into a number of triggeroutputs carrying respective divided portions of the clock pulses at asecond clock frequency equal to the first clock frequency divided by thenumber; (c) sampling the electrical signals respectively by triggeringon the divided portions, thereby generating sampled electrical signalswherein sample durations of the sampled electrical signals aresubstantially equal to widths of the clock pulses; (d) converting thesampled electrical signals to sampled optical signals by modulatingrespectively the laser transmitters with the sampled electrical signalsand outputting respectively the sampled optical signals on the opticalfibers; and (e) combining the sampled optical signals onto a singleoptical fiber. Preferably, the sampling includes operationallyconnecting the trigger outputs to the respective laser transmitters andoperationally connecting the electrical signals to the respective lasertransmitters, and more preferably, sampling includes operationallyconnecting said trigger outputs to respective anodes of the respectivelaser transmitters and operationally connecting the electrical signalsto respective cathodes of the respective laser transmitters.

According to the present invention there is provided a method, ofmultiplexing optical signals in a node of an optical network including aplurality of optical fibers, respectively transmitting a plurality ofoptical signals, including the steps of: (a) generating clock pulses ata first clock frequency; (b) dividing the clock pulses respectively intoa number of trigger outputs carrying respective divided portions of theclock pulses at a second clock frequency equal to the first clockfrequency divided by the number; (c) sampling the optical signalsrespectively by inputting the optical signals respectively into opticalmodulators and triggering the optical modulators on the dividedportions, thereby generating sampled optical signals; and (d) combiningthe sampled optical signals onto a single optical fiber.

According to the present invention there is provided a system formultiplexing downlink optical signals in a site including multiple basetransceiver stations, operationally connected to respective lasertransmitters; the laser transmitters connected to multiple opticalfibers respectively carrying the optical signals including: (a) a clockgenerating clock pulses at a first clock frequency; (b) a serial toparallel converter dividing the clock pulses respectively into a numberof trigger outputs respectively carrying divided portions of the clockpulses at a second clock frequency equal to the first clock frequencydivided by said number; (c) a plurality of laser transmittersoperationally connected respectively to said trigger outputs; whereinthe optical signals are sampled by driving the lasers on the respectivedivided portions; and (d) an optical coupler combining the opticalfibers onto a single output optical fiber carrying a multiplexed opticalsignal. The system, further includes (e) an optical receiver connectedto the output optical fiber, converting the multiplexed optical signalto a multiplexed electrical signal; (f) an RF amplifier connected to anoutput of said optical receiver, amplifying the multiplexed electricalsignal; and (g) an antenna operationally connected to said RF amplifierbroadcasting the multiplexed electrical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a prior art drawing of a conventional optical time divisionmultiplex system;

FIG. 2 is a drawing, according to an embodiment of the presentinvention, in a communications network using direct modulation lasersources;

FIG. 2 a is a drawing, according to another embodiment of the presentinvention, in a communications network using direct modulation lasersources;

FIG. 3 is a is a drawing, according to an embodiment of the presentinvention, using external modulation of multiple optical signals;

FIG. 4 is a drawing, according to an embodiment of the presentinvention, using direct modulation in a mobile telephone application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a system and method of multiplexing opticalsignals onto a single optical fiber. Specifically, the system and methodincludes sampling multiple signals and optically coupling the sampledsignals onto a single optical fiber. The present invention can bereadily used to upgrade existing networks in the field and for newinstallations of fiber optic communications networks.

The principles and operation of a system and method of multiplexingoptical signals by sampling individually multiple signals and couplingthe sampled signals onto a single fiber, according to the presentinvention, may be better understood with reference to the drawings andthe accompanying description.

It should be noted, that although the discussion herein relates tooptical multiplexing with RF analog signals, the present invention may,by non-limiting example, be alternatively configured as well usingdigital data signals.

Once a signal is multiplexed, according to an embodiment of the presentinvention, demultiplexing is achieved using any of the methods known inthe art. For moderate digital data rates, multiplexing is achieved, forinstance, using well-known clock and data recovery circuits in the timedomain. For high speeds or high frequency signals, gated opticalexternal modulators may be used.

Before explaining embodiments of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

As such, those skilled in the art will appreciate that the conception,upon which this disclosure is based, may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

Referring now to the drawings, FIG. 2 illustrates a system and methodfor multiplexing signals carried in several optical fibers into a singleoptical fiber, thereby freeing optical fibers for additional use in theoptical network. An existing optical communications network includesmultiple laser transmitters 201 a to 201 n. Laser transmitters 201 a to201 n are directly modulated respectively, by varying the current biason the laser diodes of laser transmitters 201 according to an electricalsignal generated respectively by signal generators 205 a to 205 n. Themodulated optical signals are carried by optical fibers 215 a to 215 n.The multiplexing system, according to a present invention, uses a clock207 that generates a serial clock signal, e.g., serial pulses at aprecise frequency. The clock rate, preferably according to Nyquist, isgreater than twice the number of multiplexed channels multiplied by themaximum frequency transmitted per channel. The serial clock signal isinput to a serial to parallel converter 209. Serial to parallelconverter 209, has multiple trigger outputs 221 a to 221 n, a triggeroutput 221 to each sampling module 203. According to one embodiment ofthe present invention, serial to parallel converter 209 outputs a firstpulse to sampling module 203 a, a second pulse to sampling module 203 b,and so on, cycling through all sampling modules 203 to sampling module203 n and then returning to first sampling module 203 a. On receiving aclock pulse, sampling modules 203 a to 203 n, are triggered to sampletheir respective inputs and output the signal samples to lasertransmitters 201 a to 201 n. Preferably, the time duration of the signalsamples are similar to the clock pulse width. Consequently, lasertransmitters 201 a to 201 n respectively have a “non-zero output” onlywhen sampling modules 203 a to 203 n respectively are triggered. Theterm “zero output” refers to the type of modulation used, in digitalmodulation that is unipolar, “zero output” preferably means an opticaloutput less than the laser threshold whereas in an analog output whichis bipolar a “zero output” preferably refers to a bias point near halfthe peak maximum instantaneous output level of the laser diode. Sampledoptical signals output for laser transmitters 201 a to 201 n are carriedby optical fibers 215 a to 215 n respectively. The sampled signals arecombined with an optical coupler 211. The optical time domainmultiplexed signal is carried on a single optical fiber 213 freeing n−1fibers for additional use in the network. In one embodiment of thepresent invention, sampling modules 203 a to 203 n include RF mixersthat convolute the clock output with the signal input as input to lasertransmitters 201 a to 201 n.

As an example, multiplexing 10 RF analog channels up maximum frequency100 Mhz uses preferably a clock frequency of 2 Ghz. Laser transmitters201 are preferably sampled at a frequency of 200 Mhz. The multiplesampled signals are consequently optical time division multiplexed ontoa single optical fiber carrying 2 Ghz modulation from multiple lasertransmitters.

In another embodiment of the present invention, shown in FIG. 2 a,sampling may be achieved by driving laser transmitters 201 a to 201 ndirectly with both triggering pulses and outputs from generators 205 ato 205 n without requiring intervening sampling modules 203 a to 203 n.In FIG. 2 a, laser drivers 217 a to 217 n receive clock pulses fromtrigger outputs 221 a to 221 n of serial to parallel converter 209 anddrive the respective anodes of laser transmitters 201 a to 201 n.Outputs of RF signal generators 205 a to 205 n are connected torespective attenuators 219 a to 219 n to achieve appropriate signallevels, so as not to cause excess distortion in laser transmitters 201 ato 201 n, and operationally connected to the respective cathodes oflaser transmitters 201 a to 201 n. The embodiment shown in FIG. 2 a hasan advantage over the embodiment of FIG. 2 since the embodiment of FIG.2 a requires fewer parts than the embodiment of FIG. 2 and does notrequire filters to provide isolation.

The present invention according to the embodiments shown in FIG. 2 andFIG. 2 a have an advantage that with the exception of a simple, low costand easy to install optical coupler 211, all the additional equipmentrequired for multiplexing is in the electrical domain and may added intoan existed network at relatively low cost.

Another possible configuration of the present invention is shown in FIG.3. In the embodiment of FIG. 3, signal generators 205 a to 205 n areinput to laser transmitters 201 a to 201 n. Modulated optical outputsfrom laser transmitters 201 a to 201 n are input to external modulators301 a to 301 n. The serial clock signal output from clock 207 is inputto serial to parallel converter 209. Serial to parallel converter 209,has multiple trigger outputs 221 a to 221 n, one trigger output 221 toeach external modulator 301. Serial to parallel converter 209 outputs afirst pulse to external modulator 301 a, a second pulse to externalmodulator 301 b, and so on, cycling through all the external modulators301 a to external modulator 301 n and then returning to first externalmodulator 301 a. On receiving a clock pulse, external modulators 301 ato 301 n, are triggered to sample their respective optical inputs andoutput the signal samples to optical fibers 215 a to 215 n respectively.Optical fibers 215 a to 215 n are optically combined by optical coupler211 into a single optical fiber 213.

The embodiment of FIG. 3 is appropriate for fast clock rates. Forinstance, multiplexing 5 data channels operating at 2.5 Gbs(gigabit/sec) according to the present invention, requires clock 207 tooutput pulses at a frequency at least 25 Gbs. Alternatively, anembodiment similar to that of FIG. 3 using externally modulation may beconfigured using laser transmitters with fast rise and fall times of forinstance 40 ps, characteristic of, for instance, a mode locked pulsedlaser.

Another embodiment of the present invention, shown in FIG. 4 is anapplication of optical time domain multiplexing in the field of mobilephone cellular communications. Referring to FIG. 4, base transceiverstations 405 a to 405 n are located at the same site. Base transceiverstations 405 a to 405 n belong to different cellular services.Alternatively, base transceiver stations 405 a to 405 n belong to thesame cellular service but operate at frequency channels different fromeach other. Downlink (mobile receive) signals are output throughuplink/downlink duplexers 409 a to 409 n and operationally connected,(after suitable attenuation, attenuators not shown) to lasertransmitters 201 a to 201 n. As in the embodiments shown in FIG. 2 andFIG. 2 a, clock 207 is operationally connected to respective lasertransmitters 201 a to 201 n through serial to parallel converter 209 andthrough respective laser drivers 217 a to 217 n. Optical outputs 215 ato 215 n from laser transmitters 201 a to 201 n are optically combinedin optical coupler 211; the multiplexed optical output is carried bysingle optical fiber 213. The optical signal carried by optical fiber213 is received by an optical receiver 408 and converted into amultiplexed electrical signal. The multiplexed electrical signal isconnected to an input of RF amplifier 410 and transmitted through anantenna duplexer 411 to antenna 413 for broadcasting. Mobile units 407 ato 407 n receive respectively signals from base transceiver stations 405a to 405 n. The uplink (mobile transmit) signal path from antenna 413 tobase station transceivers 405 a to 405 n is not shown in FIG. 4.

As an example, of the embodiment shown in FIG. 4, three base transceiverstations each have distinct downlink frequency bands within 800-900 Mhz.Clock 207 operates for instance at 6 Ghz. and laser transmitters 201 ato 201 n are sampled at 2 Ghz. However, RF amplifier 410 has a frequencyresponse of for instance 1 Ghz. Therefore, RF amplifier 410 is not fastenough to amplify the sampling transitions of the signals and thereforesufficiently distortion free signals from base transceiver stations 405a to 405 n are transmit from antenna 413.

Therefore, the foregoing is considered as illustrative only of theprinciples of the invention. Further, since numerous modifications andchanges will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and operationshown and described, and accordingly, all suitable modifications andequivalents may be resorted to, falling within the scope of theinvention.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

1. A method of multiplexing optical signals in a node of an opticalnetwork including as inputs a plurality of electrical signals, aplurality of laser transmitters, and as outputs a plurality of opticalfibers, comprising the steps of: (a) generating clock pulses at a firstclock frequency; (b) dividing said clock pulses respectively into a,number of trigger outputs carrying respective divided portions of saidclock pulses at a second clock frequency equal to said first clockfrequency divided by said number; (c) sampling the electrical signalsrespectively by triggering on said divided portions, thereby generatingsampled electrical signals wherein sample durations of said sampledelectrical signals are substantially equal to widths of said clockpulses; (d) converting said sampled electrical signals to sampledoptical signals by modulating respectively the laser transmitters withsaid sampled electrical signals and outputting respectively said sampledoptical signals on the optical fibers; and (e) combining said sampledoptical signals onto a single optical fiber.
 2. The method, according toclaim 1, wherein said (c) sampling includes operationally connectingsaid trigger outputs to the respective laser transmitters andoperationally connecting the electrical signals to the respective lasertransmitters.
 3. The method, according to claim 1, wherein said (c)sampling includes operationally connecting said trigger outputs torespective anodes of the respective laser transmitters and operationallyconnecting the electrical signals to respective cathodes of therespective laser transmitters.
 4. A method of multiplexing opticalsignals in a node of an optical network including a plurality of opticalfibers, respectively transmitting a plurality of optical signals,comprising the steps of: (a) generating clock pulses at a first clockfrequency; (b) dividing said clock pulses respectively into a number oftrigger outputs carrying respective divided portions of said clockpulses at a second clock frequency equal to said first clock frequencydivided by said number; (c) sampling the optical signals respectively byinputting said optical signals respectively into optical modulators andtriggering said optical modulators on said divided portions, therebygenerating sampled optical signals; and (d) combining said sampledoptical signals onto a single optical fiber.
 5. A system formultiplexing downlink optical signals in a site including multiple basetransceiver stations, operationally connected to respective lasertransmitters; the laser transmitters connected to multiple opticalfibers respectively carrying the optical signals comprising: (a) a clockgenerating clock pulses at a first clock frequency; (b) a serial toparallel converter dividing said clock pulses respectively into a numberof trigger outputs carrying respective divided portions of said clockpulses at a second clock frequency equal to said first clock frequencydivided by said number; (c) a plurality of laser transmittersoperationally connected respectively to said trigger outputs; whereinthe optical signals are sampled by driving said lasers on said dividedportions; and (d) an optical coupler combining the optical fibers onto asingle output optical fiber carrying a multiplexed optical signal. 6.The system, according to claim 5 further comprising: (e) an opticalreceiver connected to said output optical fiber, converting saidmultiplexed optical signal to a multiplexed electrical signal; (f) an RFamplifier connected to an output of said optical receiver, amplifyingsaid multiplexed electrical signal; and (g) an antenna operationallyconnected to said RF amplifier broadcasting said multiplexed electricalsignal.